This invention relates to inert rigid vertebral prosthetic devices and methods for implanting the devices between adjacent vertebrae to treat or prevent back or neck pain in patients with ruptured or degenerated intervertebral discs and for replacing vertebral bodies damaged by fracture, tumor or degenerative process.
The surgical implant devices and methods of U.S. Pat. Nos. 4,743,256; 4,834,757; and 4,878,915 have greatly improved the success rate and have simplified the surgical techniques in interbody vertebral fusion. In the procedures covered by these patents, biologically acceptable but completely inert bodies are bottomed in channels or grooves of adjoining vertebrae and receive bone ingrowth which quickly fuses the structure to the bone and forms a living bone bridge across the fusion area. The materials and methods which are disclosed in these patents are hereby incorporated by reference.
A more recent invention provides interbody fusion without cutting grooves or channels in the vertebrae and is well suited for anterior cervical and lumbar fusion. The invention of U.S. Pat. No. 5,192,327 provides ring-like prosthetic plugs or discs bottomed on end faces of adjoining vertebrae and constructed and arranged so that they can be used singly or stacked plurally to accommodate individual surgical requirements. These rings can replace excised discs and vertebrae and can also be mounted inside the fibrous disc column connecting adjoining vertebrae. The annular units are oval or partial oval shaped, to conform with vertebral disc shapes, have ridged or peaked surfaces for biting into the vertebrae on which they are seated, for receiving bone ingrowth in valleys between the peaks, and for interdigitating when stacked. The open central portion of the ring is preferably packed with bone graft or other osteobiologic materials to facilitate bone ingrowth. The methods and materials disclosed in the '327 patent are hereby incorporated by reference.
The present invention includes a plurality of inert rigid vertebral prosthetic devices ("cages") and methods for use. Each of these cages is a generally rectangular shaped body, conforming in shape and size with the hard end plates of the vertebrae on which the prosthetic device is to be seated. Each of the cages has ridged surfaces for gripping the vertebrae to resist expulsion, forming valleys to facilitate bone ingrowth, and interdigitating with adjacent cages for stacking. The cages of the current invention have ridges which run in both the anterior/posterior and medial/lateral directions respectively. Such ridges prevent slipping in the medial/lateral and anterior/posterior directions, whether implanted singly or stacked with other cages of this invention. Also, the anterior/posterior and medial/lateral ridges intersect in the corners of the surfaces to produce a crisscross pattern, which further resists rotation or other torsional type forces.
Also, each cage is provided with a plurality of threaded openings in the side walls. While it is known in the art to provide an opening for use with an insertion tool, this invention includes a plurality of additional threaded openings, at various locations around the side walls. These threaded openings are positioned and sized to receive threaded components of other spine stabilization devices. For example, in corpectomies or vertebrectomies, an anterolateral plate or rod system could be anchored to the cage by threading a connecting screw into a laterally placed hole, and the plate or rods could then be secured to adjacent vertebrae. Similarly, the cages could be used in conjunction with posterior spinal rods or plate systems. Examples of such plates and rod systems are disclosed in U.S. Pat. Nos. 4,696,290; 5,261,910; 4,887,595; 5,217,461; and 5,257,993. When posterior spinal rods are used, the connecting screws which connect the cage with the posterior osteosynthetic system act as true artificial pedicles. The devices and methods disclosed in the above cited patents are hereby incorporated by reference.
The invention comprises a set of cages of various sizes. The cages are provided in various heights for use individually. Also, several cages may be stacked to produce a prosthetic device of appropriate height to meet specific surgical demands. The ridges of the stacked cages interdigitate, and a locking screw and nut may be used to secure the stack to produce a singular rigid prosthetic device. Recesses in the ridged faces of the cages allow the locking screw to be sized such that the screw does not extend beyond the top and bottom surfaces of the stack. Thus, the screw does not interfere with placement of the stack into the vertebral column, and it does not assume any axial loading from adjacent vertebrae. Also, smaller cages may be used to replace partially damaged discs, or a stack of smaller cages may be used when vertebrae are partially excised. Additionally, wedge-shaped cages are provided for surgical applications where anatomical curvature is needed. By providing such a variety, the surgeon can choose the proper combination of cages to accommodate a myriad of different surgical situations.
The cages are preferably made of radiolucent materials such as a carbon fiber reinforced high temperature thermoplastic. Preferred polymers are polyether ether ketone, polyether ketone ketone, polyether ketone ether ketone ketone, and carbon fiber reinforced polyether ketone ether ketone ketone. Additionally, polycarbonate, polyprophylene, polyethylene, polysulfone, and polyphenylsulfone type plastic material filled with glass or carbon fibers can be used. Other orthopaedic implant materials such as stainless steel, titanium, and cobalt-chromium alloys may be used.
The invention further comprises a method of use. In this method, the vertebral column is prepared to receive the prosthetic device, a stack of at least one of the cages is formed to fit into the vertebral column, graft material is placed within an aperture in the stack, and the stack is inserted into the vertebral column. Following insertion, one or more connecting screws may be used to secure the stack to other spinal stabilization devices, such as plates or rods.